1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator (TCXO), and more particularly to a temperature-compensated crystal oscillator with reduced phase noise.
2. Description of the Related Art
Temperature-compensated crystal oscillators are used as a reference frequency source in mobile communication devices such as cellular phone terminals or the like, for example, because they are capable of compensating for the frequency vs. temperature characteristics thereof due to the crystal unit for increased frequency stability. In recent years, there has been a demand for a temperature-compensated crystal oscillator with reduced phase noise for the purpose of maintaining desired communication quality in digital communications.
FIG. 1 shows a circuit arrangement of a conventional temperature-compensated crystal oscillator. As shown in FIG. 1, the conventional temperature-compensated crystal oscillator generally comprises a crystal oscillator and a temperature compensating mechanism, which are integrated in an IC (integrated circuit) chip. The crystal oscillator has crystal unit 1 as an inductor and a pair of voltage-variable capacitive elements 2a, 2b connected respectively to the opposite ends of crystal unit 1, the voltage-variable capacitive elements 2a, 2b doubling as oscillating capacitors. Crystal unit 1 comprises, for example, an AT-cut quartz-crystal blank whose frequency vs. temperature characteristics is represented by a cubic curve nearly at the room temperature. Each of voltage-variable capacitive elements 2a, 2b typically comprises a variable-capacitance diode. Voltage-variable capacitive elements 2a, 2b and crystal unit 1 jointly make up a resonance circuit. Voltage-variable capacitive elements 2a, 2b have respective anodes connected to a ground potential as a reference potential and respective cathodes to which there is applied a temperature compensating voltage Vc via respective resistors 6a, 6b which serve to cut off high frequency components.
Inverting amplifier 4 with feedback resistor 3 connected thereacross is connected across crystal unit 1 for amplifying the resonance frequency component of the resonance circuit. Inverting amplifier 4 should preferably comprise a CMOS (Complementary Metal Oxide Semiconductor) inverter. DC-blocking capacitors 5a, 5b are provided respectively to input and output terminals of inverting amplifier 4. The temperature-compensated crystal oscillator produces an output voltage Vo from the junction between capacitor 5b and crystal unit 1. The temperature-compensated crystal oscillator may be summarized as a crystal oscillator with voltage-variable capacitive elements 2a, 2b inserted in its closed oscillation loop. The frequency vs. temperature characteristics of the crystal oscillator is represented by a cubic curve because of the characteristics of the crystal unit.
The temperature compensating mechanism generates a low-level detected-temperature signal in response to the ambient temperature based on, for example, the temperature vs. resistance characteristics of a resistor in the IC chip, and generates the temperature compensating voltage Vc from a constant voltage source based on or amplifying the detected-temperature signal. As shown in FIG. 2, the temperature compensating voltage Vc has temperature vs. voltage characteristics having a reference voltage Vco at 25xc2x0 C., which corresponds to the frequency vs. temperature characteristics of the crystal oscillator, and is represented by a cubic curve superposed on the reference voltage Vco. Various circuits for generating the temperature compensating voltage Vc are known to those skilled in the art.
When the temperature compensating voltage Vc from the temperature compensating mechanism is applied to the cathodes of voltage-variable capacitive elements 2a, 2b, the capacitances across these voltage-variable capacitive elements change. Since the equivalent series capacitance as viewed from crystal unit 1 also changes, a change in the frequency vs. temperature characteristics of the crystal oscillator can be compensated for and the frequency vs. temperature characteristics can be made flat by thus applying the temperature compensating voltage Vc which corresponds to the frequency vs. temperature characteristics of the crystal oscillator.
The temperature compensating voltage Vc is of a positive potential to apply a reverse voltage to the cathodes of voltage-variable capacitive elements 2a, 2b, so that the capacitances across voltage-variable capacitive elements 2a, 2b will be reduced in inverse proportion to the applied voltage. That is, the capacitances across voltage-variable capacitive elements 2a, 2b are changed by the reverse voltage applied thereto, with no current flowing between the anodes and cathodes of voltage-variable capacitive elements 2a, 2b. 
However, the above crystal oscillator suffers the following problems by applying the temperature compensating voltage Vc to the cathodes of voltage-variable capacitive elements 2a, 2b. As shown in FIG. 3, the output signal Vo of the temperature-compensated crystal oscillator has a waveform represented by a high-frequency voltage superposed on the temperature compensating voltage Vc. The waveform of the output signal Vo has an upper limit Vmax and a lower limit Vmin. In order to keep the cathode of the voltage-variable capacitive element positive with respect to the anode thereof, the lower limit Vmin of the output signal Vo has to be greater than the potential (0 V in FIG. 3) of the anode. Therefore, the temperature compensating voltage Vc which is typified by the reference voltage Vco needs to be of a value depending on the amplitude level of the output signal, i.e., a value with the half value (Vmax+Vmin)/2 of the amplitude level being added as an offset voltage thereto. However, as described above, the temperature compensating voltage Vc is generated from the constant voltage source based on or amplifying the low-level detected-temperature signal according to the temperature vs. resistance characteristics of the resistor. The voltage signal generated by the constant voltage source generally contains more noise as the voltage value thereof is higher. Consequently, since the conventional temperature-compensated crystal oscillator needs to set the temperature compensating voltage Vc (or the reference voltage Vco) to a higher value depending on the output signal thereof, the output signal contains large noise, deteriorating the phase noise characteristics of the temperature-compensated crystal oscillator.
It is therefore an object of the present invention to provide a temperature-compensated crystal oscillator with good phase noise characteristics.
According to the present invention, the above object can be achieved by a temperature-compensated crystal oscillator comprising a crystal unit having frequency vs. temperature characteristics, a voltage-variable capacitive element inserted in a closed oscillation loop including the crystal unit, an amplifier for keeping oscillation in the closed oscillating loop, means for applying a temperature compensating voltage to an anode of the voltage-variable capacitive element, and means for applying a voltage to prevent a current from flowing through the voltage-variable capacitive element to a cathode of the voltage-variable capacitive element, whereby the frequency vs. temperature characteristics can be compensated for by the temperature compensating voltage applied to the anode of the voltage-variable capacitive element.
Because the temperature compensating voltage is applied to the anode of the voltage-variable capacitive element, the voltage applied to the cathode of the voltage-variable capacitive element is relatively increased to apply a reverse voltage to the voltage-variable capacitive element, thus preventing a direct current from flowing through the voltage-variable capacitive element. As a result, the temperature compensating voltage can be set to a desired value without having to take into account the amplitude level of a high-frequency output voltage (oscillated output) of the temperature-compensated crystal oscillator. Since it is not necessary to add an offset voltage (Vmax+Vmin)/2 to the temperature compensating voltage and hence the temperature compensating voltage can be lowered, noise contained in the temperature compensating voltage can be reduced. As a result, the phase noise characteristics of the temperature-compensated crystal oscillator is maintained at a good level.